A glass-ceramic is a glass that has been transformed into a solid with a high crystalline component in its microstructure. Most glasses, composed of more than 50% Si0.sub.2 and other oxides, are super-cooled liquids (i.e. amorphous solids with the structure of a liquid). The absence of porosity normally found in ordinary ceramics, makes glass-ceramics strong and the inherent mixed structure makes low coefficients of thermal expansion possible.
The construction of complex components made from glass-ceramics is a difficult and expensive process. However, the advantages glass-ceramics offer, like excellent mechanical and thermal stability, often outweigh the manufacturing drawbacks. The present invention reduces or eliminates some of the manufacturing drawbacks or creating glass ceramic parts and enhances the desirability and advantages that glass-ceramics offer.
Presently, parts made from commercially available quartz-like materials, such as Cervit, Zerodur, fused silica, or glasses such as Schott Optical's BK-7 and other types of glass-ceramics, are constructed from a number of techniques including milling, boring, drilling, grinding, sanding, lapping, polishing, diffusion and lamination. Because it is difficult to produce large and complicated monolithic parts from such materials, smaller, less complicated structures are typically glued, bonded or optically sealed together in order to achieve larger complex structures.
Prior art techniques use a set of multi-stage operations to prepare complex monolithic structures. Each technique accomplishes a specific goal during the manufacturing of a structure from a blank of the unfinished material. Milling, boring and drilling serves to shape and form the ceramic into a useful shape.
For example, the successful construction of a gas filled ring laser angular rate sensor requires boring a laser cavity into a block of Cervit or Zerodur. Typically, three or more interconnected tunnels are bored within the block. Grinding, lapping and polishing serve to shape or form the surfaces of milled, bored, or drilled block features. For example, the successful preparation of the mirror structure mating surfaces at the corners of the laser gyro block can only be accomplished by grinding, lapping or polishing the surface. Mirror assemblies are bonded to mating surfaces at the corners of the block.
Ring laser angular rate sensor components may be constructed using the techniques of the invention. Prior art methods of creating ring laser angular rate sensors using a glass block created from the methods of the prior art are discussed above. In some cases, the resulting laser block has a tendency to out gas impurities from the surface of the machined cavities Prior art ring angular rate sensors block glass has tended to fracture because the glass has low thermal conductivity. Prior art laser gyro blocks must be cleaned by very harsh chemicals which have a tendency to etch the glass material of the block to remove fractures.
Due to the relatively low melting point and low coeficient of expansion of prior art glasses the types of commercial frits that may be used on the glasses have been limited. Prior art glass also suffers from the problem that the characteristics of the material are relatively undynamic and cannot be varied to fit each application. Also, prior art glass looses its memory after being exposed to excessive heat which leads to different coefficients of expansion. This in turn ruins the optical properties of the glass for its intended application. Prior art blocks also have a tendency to be relatively expensive. As a result, prior art ring laser gyroscopes built from the production method of the prior art tend to be limited in their amount of acceleration stress that the block can withstand. Also, prior art blocks cannot be cleaned with high-temperature processes that exceed 1000 degrees.
The method of the invention depends on electro-discharge machining which is a process of utilizing a conductive material and a high electric field to machine an object. Electro-discharge machining is well known in the art. A good reference material containing background material on electro-discharge machining can be found in Machine Tool Practices, Third Edition, Richard R. Kibbe, et al. John Wiley and Sons, New York 1987. pp. 739-740.
Electro-discharge machining is a non-traditional machining process involving the use of electric sparks. Electro-discharge machining removes material through the use of an electric spark generated by high energy power supply. Electro-discharge machining is known as EDM. EDM works by eroding material from a part by bringing the part close to a large potential voltage difference. The traveling of electrons from the electrode to the part erodes the part away. EDM is similar to other methods of machining in that it requires part positioning and measurement. EDM has been applied to numerical control systems thus increasing the effectiveness and precision of the results. One additional aspect of the EDM process is that the shape of the electrode dictates the features imparted in the part features eroded in the part. Electro-discharge machining electrodes are traditionally made for metal or graphite. An allied method is wirecut EDM where a wire is used as an electrode and the types of part geometry that may be machined are more intricate. The wirecut EDM works similarly to a band saw in conventional techniques.
Some ring laser gyro mirror assemblies themselves are composed of a number of glass-ceramic parts including the optical path length control (PLC) transducer mirror. The PLC transducer mirror often comprises two pieces of glass-ceramic having a common mating surface. In the current method of manufacturing PLC transducers two pieces of worked glass-ceramic are mated and bonded with an optical seal. Alternately, a bonding agent deposited on the mating surfaces such as a gold diffusion layer or a thin film of glass is used. A good discussion of such bonding techniques for transducer mirrors may be found in U.S. Pat. No. 4,865,451 to Ahonen, et al. entitled "SILICON SUBSTRATE MIRROR ASSEMBLY FOR LASERS".
Multi-layer structures manufactured in accordance with known techniques discussed hereinabove suffer from a number of disadvantages. For example, the use of a gold diffusion bond appears to result in an "aging bond," wherein bonding strength degrades with time. Further, the diffusion bonding process requires very high temperatures and pressures which may degrade the mechanical and thermal stability of the glass-ceramic material. Similarly, the use of epoxy seals, indium seals and other sealing and bonding materials result in structures with poor mechanical, electrical or optical properties.
Referring now to FIGS. 1A-1E which shows the processing steps illustrating one prior art method of creating glass-ceramic structures. The objective of the process is to create a final compound assembly using a glass-ceramic material.
As shown in FIGS. 1A and 1B, a glass-ceramic block 100 is first milled using the techniques described above. The milled surface on glass-ceramic block 102 are indicated by 101 where a milling machine has taken off glass at a slow rate to achieve a first subassembly structure 104 as shown in FIG. 1C. Referring now to FIG. 1D, the first subassembly structure 104, a second subassembly structure 106 and a gold diffusion layer 108 is shown. The second subassembly structure 106 is fabricated in the same manner as the first subassembly structure 104. In this example, a simple compound assembly is fabricated from two cylindrical disks bonded together. The bonding technique used is a traditional gold diffusion bond which is created under high temperature. Two subassemblies 104 and 106 are coated with the gold diffusion layer 108 and aligned along lines 110. The parts are then brought into intimate contact, pre-loaded and the temperature is raised. The increase in temperature is detrimental to the properties of the glass-ceramic as discussed above but is crucial to the construction of a bonded assembly. In the final assembly step shown in FIG. 1E, the part is created and subassembly 104 is joined to subassembly 106 by the gold diffusion bond 108. Of course, the gold diffusion bond may be replaced by using glass frits, or brazing.
The difficulty and expense of prior art methods has created a need for a new method of manufacturing glass-ceramic structures. The present invention eliminates or reduces the need for extensive machining, drilling or boring of a ceramic subassembly by working with the material in the green state and using electro-discharge machining to fashion the glass.